Friction stir spot welding is a method of spot-joining two or more metal layers of a workpiece together. A cylindrical punch with a shouldered probe at its tip is rotated and driven into the workpiece layers to be joined. Friction between the probe and the workpiece layers causes the layers to soften and plasticise without melting, and the rotation of the probe displaces the material and causes the plasticised portions of the two layers to intermingle. When the punch is withdrawn and the workpiece allowed to cool, the intermingled plasticised portions harden and produce a welded joint between the two layers.
Recently, the softening of material through a friction stir mechanism has been used in relation to blind riveting. Rather than pre-drilling a hole for the rivet, the rivet is rotated and driven into the workpiece so as to cause friction stir softening of the material. The rivet is then driven through the workpiece and once the material has cooled, the rivet is upset in the usual manner.
Present friction stir spot-joining machines and methods which utilise friction stir softening, such as those above, use an actuator (a motor) to produce the required rotation of the blind rivet or welding punch. A second actuator is used to produce the motion required to either drive the tip of the punch into the workpiece, or to pull on the mandrel of a blind rivet to upset it, depending on the process in question. Use of two actuators not only increases the cost of the apparatus, but also significantly increases its bulk and weight. The added weight may limit the choice of robotic arms on which the apparatus can be mounted due to some arms not being able to sustain the force necessary to lift the apparatus, and the added bulk may prevent the apparatus from reaching into small spaces. Use of two actuators may also adversely affect the service life and/or maintenance cost of the apparatus.
It is an object of the present invention to obviate or mitigate at least one of the aforesaid disadvantages, and/or to provide an improved or alternative friction stir blind riveting apparatus or an improved or alternative method of performing friction stir blind riveting.